The object of the proposed research is to develop and test a simple technique to be used for determining the cell cycle stage when cultured cells are sensitive to various inhibitory agents (drugs, antibiotics, antimetabolites) and lesions in mutants conditionally defective in cell cycle functions. The most important feature of the proposed technique is that it can be used to determine the cell cycle properties of cells in asynchronous culture. The technique basically involves an analysis of the amount of cell division or cell cycle marker substance that is produced after administration of a growth inhibiting agent or after shifting conditional cycle-blocked mutants to growth restricting conditions. The extent to which cells will grow or express specific cell cycle events indicates important features about their periods of sensitivity during the cell cycle. The proposed techniques will be used to examine the regulation of cell cycle events. We plan to determine what events control the immediate progress of the cell cycle and what interdependency relationships are found among various cell cycle events.